A poppet type or spool type electromagnetic valve is known as a general pressure controller and the electromagnetic valve blocks (or opens) the channel by energizing a valve element with energizing force of a spring in one direction and opens (or blocks) the channel by moving the valve element with energization of a solenoid in another direction against energizing force of the spring. Since such an electromagnetic valve cannot control pressure by moving a valve element when a power supply feeding a current value to a solenoid is defective, the electromagnetic valve has a so-called fail-safe function.
Electromagnetic valves having the fail-safe function include those described in Patent Documents 1 and 2 listed below.
An electromagnetic proportional decompression valve described in Patent Document 1 is provided with a spool for causing a control pressure port to generate a pressure in accordance with communicative connection between a supply port and a tank port and also provided with a first solenoid plunger for pressing against an end face of the spool and a second solenoid plunger for pressing against an end face of the first solenoid and, when a current is shut off, the fail-safe function under abnormal conditions is fulfilled by retaining the first solenoid plunger at a predetermined position by being pressed by the second solenoid plunger with energizing force of a spring to maintain a predetermined control pressure even when a solenoid exciting current is shut off.
Also, an electromagnetic valve described in Patent Document 2 can operate a spool with energizing force of a compression spring to communicate an input port and an output port and also makes a drain port and the output port communicable by operating the spool against energizing force of the compression spring with electromagnetic force and, when the spool does not operate due to a coil defect, the output port is made controllable to open/close even when a defect appears by supplying a predetermined oil pressure to an oil pressure supply mechanism to move the spool against energizing force of the compression spring to change the valve opening state of the output port.
Patent Document 1: Japanese Patent No. 3110861
Patent Document 2: Japanese Patent Application Laid-Open No. 2004-060806